Preparation of 2,2-dihydrocarbyl-3-propiolactones

ABSTRACT

2,2-Dihydrocarbyl-3-propiolactones are prepared by the thermolysis at 175° C. to 350° C. of low molecular weight (oligomeric) poly(3-hydroxy-2,2-dihydrocarbylpropionic acid) that contains carboxyl ends groups, in the presence of alkali metal, ammonium or phosphonium cations. The propiolactones may be polymerized to high molecular weight polymers.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The preparation of 2,2-dihydrocarbyl-3-propiolactones by thermolysis ofoligomeric 3-hydroxy-2,2-dihydrocarbylpropionic acid that contains acarboxyl end, in the presence of selected cationic catalysts isdisclosed.

2. TECHNICAL BACKGROUND

The general concept of heating the polyesters of hydroxy acids toprepare lactones, and particularly caprolactones (forming seven-memberedrings) has been known for a long time, see for example W. H. Carothers,et. al., J. Am. Chem. Soc., vol. 54, p. 761-772 (1932) and referencestherein. A more recent reference is, for example, U.S. Pat. No.4,550,181, in which polycaprolactone is heated in the presence of waterand a hydrogen halide to form caprolactone. However, preparation oflactones with four ring atoms, 3-propiolactones, is more difficult bythese methods because of the propiolactone's greater tendency to undergothermal degradation [see for example T. L. James and C. A. Wellington,J. Am. Chem. Soc., vol. 91, p. 7743-7746 (1969)].

Japanese Patent Application 48-30067 describes the preparation ofepisilon-caprolactone by the thermolysis of a polycaprolactone oligomerhaving a degree of polymerization of 2 to about 20. However, the processis carried out with the ester of the oligomeric caprolactone or in thepresence of an alcohol. The application states that it is preferable"that the carboxyl groups derived from these free acids be esterified"(from the translation). Thus free carboxyl groups are undesirableaccording to this Japanese Application.

U.S. Pat. No. 3,751,435 describes the thermolysis above 245° C. ofpoly(pivalolactone) in the presence of certain basic substances orneutral salts to give pivalolactone (2,2-dimethyl-3-propiolactone). Nomention is made of the degree of polymerization of thepoly(pivalolactone) used or the end groups it contains.

I. Luderwald, Makromol. Chem., vol. 182, p. 867-871 (1981), reports thethermolysis of poly(pivalolactone) in the presence of certain metalsalts. No mention is made of the original degree of polymerization, orthe end groups of the poly(pivalolactone) used.

L. E. Manring, et. al., Macromolecules, vol. 23, p. 1902-1907 (1990),reports that oligomers of poly(2-methyl-2-n-propyl-3-propiolactone)containing carboxyl ends, in the presence of certain cations, thermolyzefaster than higher molecular weight polymers, or polymers that do notcontain carboxyl ends.

It is an object of this invention to provide a method for thepreparation of 2,2-dihydrocarbyl-3-propiolactones by the thermolysis ofoligomers of the corresponding hydroxy acids having a relatively lowdegree of polymerization, and which contain carboxyl(ate) ends, in thepresence of certain cations. This combination of conditions allows theuse of lower temperatures and/or shorter residence times for thethermolysis, which in turn results in superior yields of the desiredpropiolactone.

SUMMARY OF THE INVENTION

This invention concerns a process for the preparation of2,2-dihydrocarbyl-3-propiolactones, comprising, heating at a temperatureabout 175° C. to about 350° C.,

(a) a mixture of ##STR1## with a compound of the formula MY; or

(b) an oligomer of the formula ##STR2## wherein:

each R¹ is independently hydrocarbyl;

n is 2 to about 200;

M is a cation selected from the group consisting of alkali metalcations, R² ₄ P⁺, and R² ₄ N⁺ ;

Y is an anion whose conjugate acid has a pKa of more than about 2 inwater; and

each R² is independently hydrocarbyl.

Also provided is a process comprising the additional step ofpolymerizing the 2,2-dihydrocarbyl-3-propiolactone that is formed.

DETAILS OF THE INVENTION

The process described herein involves the thermolysis of an oligomer ofa 3-hydroxy-2,2-dihydrocarbylpropionic acid to form the correspondinglactone. The oligomer may be made by the condensation polymerization ofthe propionic acid, a process known to those skilled in the art. Forinstance, see Experiment 1 herein, G. Odian, Principles ofPolymerization, John Wiley & Sons, New York, 1981, p. 102-105, and F. W.Billmeyer, Jr., Textbook of Polymer Science, Third Ed., John Wiley &Sons, New York, 1984, p. 26-40, both of which are hereby included byreference. Some processes for making the oligomer, particularly thecondensation of the hydroxy acid, may give some cyclic low oligomers asproducts. These cyclic compounds do not undergo the thermolysis readily.However, they may be volatile enough to be recovered in the thermolysisas a vapor along with the propiolactone. When separated from thepropiolactone, they may be recycled back to the corresponding hydroxyacid.

The two hydrocarbyl groups (R¹) (originally in the two position of themonomer) are independently chosen. It is preferred if each group R¹independently contains 1 to about 8 carbon atoms. It is also preferredif each R¹ is independently phenyl or alkyl containing 1 to about 8carbon atoms, and more preferred if each R¹ is independently an alkylgroup containing 1 to 4 carbon atoms. Especially preferred combinationsof the groups R¹ are dimethyl and methyl with n-propyl.

The degree of polymerization (sometimes abbreviated herein as DP), asused herein, means the average number of monomer units in the oligomermolecule. Thus the degree of polymerization equals n+1. It is preferredif n is 5 to about 100, and more preferred if n is about 10 to about 75.

The cation M is added as a compound, in which conjugate acid of theanion Y has a pKa of about 2 or more in water. Useful anions Y include,but are not limited to, bicarbonate, acetate, formate, benzoate,carbonate, hydroxide, and phenoxide. [Compounds that contain relativelybasic anions (the pKa of its conjugate acid is about 7 or more), such ascesium hydroxide, may actually cleave the oligomer chains randomlyinstead of at the ends, but the effect is limited, since on the firstcleavage, the anion is destroyed. The degree of polymerization of theoligomer used in the instant process shall be the degree ofpolymerization after such cleavage.] Alternatively, the cation may bepresent as the salt of the carboxyl end group of the oligomer. It isbelieved that if the cation is added as the compound MY, during theprocess, at least some of the cation is converted to the salt of thecarboxyl end groups present.

Although only a catalytically effective amount of cation need bepresent, it is preferred if the molar amount of cation present is in atleast approximately equimolar amounts to the amount of carboxyl endgroup present, and if added as the compound MY, it is more preferred ifabout 1% to about 30% excess M on a molar basis, compared to carboxylgroups, be present.

The alkali metal (lithium, sodium, potassium and cesium) cations arepreferred for M, potassium and cesium are more preferred, and cesiumcation is especially preferred. It is believed the cation R² ₄ P⁺ isrelatively unstable at process temperatures, and may catalyze thedecomposition of the propiolactone product. These problems are believedto be even more severe with R² ₄ N⁺, although in both cases, the desiredproduct is still obtained. Preferred R² groups are alkyl groupscontaining 1 to 6 carbon atoms.

In order to minimize degradation of the desired product, the processshould be carried out at as low a temperature as possible commensuratewith obtaining a reasonable reaction rate. The preferred processtemperature is about 190° C. to about 275° C., and a more preferredtemperature is about 200° C. to about 240° C. Even if temperatures atthe higher end of the range are used in the process, the improvement inreaction rate afforded by the conditions enumerated herein providehigher product yields, due to lower residence times. (These concernshave also been addressed in U.S. Pat. No. 3,751,435, which is herebyincorporated by reference).

Also important to minimize product degradation is removal of the productfrom the hot reaction zone as rapidly as possible. In order to do this,the process is preferably carried out under vacuum, to help effect rapidremoval of the desired propiolactone from the reaction matrix. The useof certain types of apparatus, such as so-called thin or wiped filmreactors, also facilitates rapid reaction and removal of the product,and is preferred. Such equipment is known to those skilled in the art,see for example U.S. Pat. No. 4,556,324, which is hereby included byreference.

Normally the product propiolactone will exit the process (thermolysis)as a vapor, particularly if a vacuum is applied. The vapor may becondensed and cooled, and if higher purity is desired, the product maybe distilled (or recrystallized in the case of solids).

The above described process may also comprise the further step ofpolymerizing the 2,2-dihydrocarbyl-3-propiolactone to the correspondingpolyester, which is a polymer comprising the repeat unit ##STR3## Suchpolymerizations are known to those skilled in the art, for instance, seeExample 2 herein, and D. B. Johns, et. al., in K. J. Ivin and T.Saegusa, Ring-Opening Polymerization, Vol. 1, Elsevier Applied SciencePublishers, Ltd., Barking, Essex, England, 1984, all of Chap. 7, butparticularly p. 468-500; all of Chap. 7 is hereby incorporated byreference.

In the following examples, the hydroxy acid, oligomer and productpropiolactone are substituted in the 2 position with a methyl and ann-propyl group, so that the propiolactone obtained is2-methyl-2-n-propyl-3-propiolactone, which is sometimes abbreviated MPin the Examples.

DESCRIPTION OF THE DRAWING

The FIGURE shows relative mass loss with time for three samplesdescribed in Example 3.

EXPERIMENT 1

To a 1L stirred stainless steel autoclave was added 684 g of2-methyl-2propyl-3-hydroxypropanoic acid. With stirring and a slow N₂purge, the vessel was heated as follows: room temperature to 180° C. for4 hours; to 200° C. for 4 hours. The vessel was cooled to 100° C. and 5ml of tetraisopropyl titanate was added. With stirring and a slow N₂purge, the vessel was heated again as follows: 100° C. to 210° C. for 4hours; to 220° C. for 4 hours; to 230° C. for 24 hours. Gel permeationchromatography of the resulting oligomer indicated it had a DP ˜16. HPLCanalysis indicates that it contains 12-15% cyclic oligomers.

EXAMPLE 1

Solid oligomer from Experiment 1 (27 g) and cesium acetate (1 g) weremixed by pulverizing them in a blender. The resulting fine powder wasdried in a 200 ml round bottom flask at 0.5 mmHg for 72 hours.

A series of experiments were done by placing 1 g of the mixture in a 1Lflask, which was placed in a kugelrohr (Aldrich Chemical Co., Model#Z101046-3) with a 500 ml receiving flask. A dry ice/acetone cooled trapwas placed after the 500 ml receiver to collect material more volatilethan MP. The kugelrohr was heated from room temperature to the desiredtemperature (Table 1) over a 1/2 hour period and then held at thedesired temperature for the time noted in Table 1.

EXAMPLE 2

A sample of oligomer was prepared by ring opening polymerization (ratherthan condensation as described in Experiment 1) to insure the initialabsence of cyclic oligomers. Polymerization was initiated bytetrabutylammonium 3-hydroxy-2-methyl-2-propyl propanoate in refluxingtetrahydrofuran. Initiator solutions were prepared by the addition ofequimolar amounts of tetrabutylammonium hydroxide and3-hydroxy-2-methyl-2-propyl propanoic acid to toluene followed byazeotropic removal of the methanol and water. The final volume wasadjusted to give a 0.2M solution of initiator in toluene. Polymerizationwas carried out as follows. To a 250 ml RB flask was added 100 mltetrahydrofuran, 15.4 g (0.12 moles) 2-methyl-2-propyl-3-propiolactoneand 20 ml of 0.2M initiator. The solution was heated at reflux (˜30minutes) until no lactone was detected by IR (1834 cm⁻). The solutionwas evaporated to dryness in a nitrogen stream yielding 15.45 g solid.The tetrabutylammonium carboxylate end groups were exchanged with excessacetic acid in THF.

Solid oligomer (15 g) and cesium acetate (0.48 g) were mixed and driedas described in Example 1.

A 1 g sample of the mixture was heated in a kugelrohr at 220° C. for 15minutes as described in Example 1. Only MP (no cyclic trimer) wascollected in the receiver. The observed yield of 0.08 g was ˜85% oftheoretical.

EXAMPLE 3 Effect of DP on the Relative Rate of Thermolysis

To oligomer samples, prepared as in Example 2, of various molecularweights (0.100 g dissolved in 1 ml methylene chloride) was added equalamounts of cesium acetate (0.100 ml of a 0.2M solution in methanol). Thesolvents were removed under vacuum. A ˜5 mg sample was heated at 5°C./min and the temperature of peak weight loss (first derivative of massloss) noted. The weight loss is due to conversion of oligomer to MP. Theresults are shown in Table 2. With cesium acetate catalyst, thethermolysis occurs at a lower temperature for lower DP oligomer. Theresults in Table 2 confirm that methyl ester terminated oligomerundergoes conversion to MP at a higher temperature than a carboxylterminated oligomer.

                  TABLE 1                                                         ______________________________________                                        Set    Time at             Mass in     Mass                                   Temper-                                                                              set temper-                                                                             Mass left in                                                                            Receiver    in                                     ature  ature     1 L flask (% MP/% trimer)                                                                           Trap                                   ______________________________________                                        200° C.                                                                       60 min    0.43 g    0.45 g      0.06 g                                                            92/7                                               200° C.                                                                       10 min    0.31 g    0.55 g      0.08 g                                                            91/8                                               230° C.                                                                       10 min    0.16 g    0.61 g      0.03 g                                                            93/6                                               240° C.                                                                       30 min    0.17 g    0.63 g      0.08 g                                                            97/3                                               ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        DP          Temperature °C.                                            ______________________________________                                         50         288                                                               100         297                                                               200         300                                                               500         315                                                               5330        320                                                                50*        315                                                               ______________________________________                                         *End groups are methyl ester                                             

Three samples were prepared using different molecular weight oligomer,made as in Example 2. Sample 1; 289 mg of DP=610 and 3.7×10⁻⁶ moles oftetrabutylphosphonium acetate (one equivalent per polymer chainequivalent) were dissolved in 4 ml CH₂ Cl₂. The solvent was removedunder reduced pressure. Sample 2; 293 mg of DP=206 and 1.1×10⁻⁵ moles oftetrabutylphosphonium acetate (one equivalent per polymer chainequivalent) were dissolved in 4 ml CH₂ C₂. The solvent was removed underreduced pressure. Sample 3; 242 mg of DP_(o) =51 and 3.7×10⁻⁵ moles oftetrabutylphosphonium acetate (one equivalent per polymer chainequivalent) were dissolved in 4 ml CH₂ Cl₂. The solvent was removedunder reduced pressure. Each sample (˜5 mg) was heated to 200° C. andmass loss monitored. The FIGURE shows relative mass loss with time foreach sample.

Eighteen mg of sample 1 was placed in a reaction tube which wasconnected to a trap by 1/16-inch diameter stainless steel tubing. WithN₂ flowing through the reaction tube and trap, the reaction tube washeated to 230° C. while the trap was cooled in liquid N₂. After 1 hourheating, the material in the reaction tube had mostly disappeared andthe trap contained predominantly MP.

EXAMPLE 4

A 27.0 g sample of the solid oligomer from Experiment 1 was ground in ablender with 1.0 g cesium acetate, then dried overnight at high vacuum.This mixture was pressed to a 13 mm pellet in a mold and placed in thefeed hopper of a batch thin film reactor (diagram attached) constructedaccording to the principles of U.S. Pat. No. 4,556,324 except that therotors of the batch reactor were arranged at the same angularorientation rather than in the sawtooth stepped helix required in thecontinuous model.

The reaction was then purged with nitrogen, evacuated to 0.5 mm Hg andheated to 275° C. The reaction was run for 17 min during which time thepressure rose to 2 mm then fell, stabilizing at about 0.9 mm. Thereactor was then cooled and opened. Product (10.7 g) was recovered fromTrap 1 which had been cooled with solid carbon dioxide. Nmr analysisindicated that it was composed of 38% 2-methyl-2-propyl-3-propiolactoneand 58% 2-methyl-1-pentene.

EXAMPLE 5

In a bottle with 10 cm diameter×10 cm cylindrical wall and 24×40 mmstandard taper outlet was placed ten 95 mm×10 mm diameter glass rods and4.0 g oligomer of Experiment 1 which had been ground with 0.12 g cesiumacetate. The bottle was placed in a standard "Kugelrohr" heater (Aldrich#Z13196-2) and assembled via standard taper joints to a 500 ml singlebulb received flask cooled with ice, thence through a reciprocatingdrive (to rotate the Kugelrohr through a 180° angle) to a solid CO₂cooled trap. The apparatus was evacuated to 0.15 mm Hg and heated to220° C. for 1 hr. There was collected in the single bulb receiving flask2.14 g 2-methyl-2-propyl-3-propiolactone, 96.5% as assayed by gc(Hewlett Packard 5890 Series II gas chromatograph with flame ionizationdetector and equipped with a J&W Scientific Co. #122-1031 30M silicacapillary column, an HP7673 automatic injector and HP3396 Series IIintegrator). There was also collected in the trap 0.44 g productconsisting of 55.2% 2-methyl-1-pentene and 42.2%2-methyl-2-propyl-3-propiolactone.

Although preferred embodiments of the invention have been describedhereinabove, it is to be understood that there is no intention to limitthe invention to the precise constructions herein disclosed, and it isto be further understood that the right is reserved to all changescoming within the scope of the invention as defined by the appendedclaims.

What is claimed is:
 1. A process for the preparation of2,2-dihydrocarbyl-3-propiolactones, comprising, heating at a temperatureabout 175° C. to about 350° C.,(a) a mixture of ##STR4## with a compoundof the formula MY; or (b) an oligomer of the formula ##STR5## whereineach R¹ is independently hydrocarbyl; n is 2 to about 200; M is a cationselected from the group consisting of alkali metal cations, R² ₄ P⁺, andR² ₄ N^(+;) Y is an anion whose conjugate acid has a pKa of more thanabout 2 in water; and each R² is independently hydrocarbyl.
 2. Theprocess as recited in claim 1 wherein each R¹ independently contains 1to about 8 carbon atoms.
 3. The process as recited in claim 1 whereineach R¹ is independently phenyl or an alkyl group containing 1 to about8 carbon atoms.
 4. The process as recited in claim 3 wherein each R¹ isindependently an alkyl group containing 1 to 4 carbon atoms.
 5. Theprocess as recited in claim 1 wherein said M is an alkali metal cation.6. The process as recited in claim 5 wherein said M is potassium orcesium.
 7. The process as recited in claim 6 wherein said M is cesium.8. The process as recited in claim 2 wherein said M is an alkali metalcation.
 9. The process as recited in claim 4 wherein said M is an alkalimetal cation.
 10. The process as recited in claim 9 wherein said M iscesium.
 11. The process as recited in claim 1 wherein said n is 5 toabout
 100. 12. The process as recited in claim 11 wherein said n isabout 10 to about
 75. 13. The process as recited in claim 8 wherein saidn is 5 to about
 100. 14. The process as recited in claim 9 wherein saidn is 5 to about
 100. 15. The process as recited in claim 1 wherein saidtemperature is about 190° C. to about 275° C.
 16. The process as recitedin claim 15 wherein said temperature is about 200° C. to about 240° C.17. The process as recited in claim 14 wherein said temperature is about190° C. to about 275° C.
 18. The process as recited in claim 1 whereinsaid cation is present in at least approximately equimolar amounts withthe carboxyl end group.
 19. The process as recited in claim 17 whereinsaid cation is present in at least approximately equimolar amounts withthe carboxyl end group.
 20. The process as recited in claim 1 whereineach R² group is independently an alkyl group containing 1 to 6 carbonatoms.